1. Field of the Invention
The present invention relates to a hydraulic continuously variable transmission. More particularly, the present invention relates to a hydraulic continuously variable transmission having a structure in which hydraulic fluid, discharged from an axial plunger pump, is supplied to an axial plunger motor through a closed hydraulic circuit, to rotate the axial plunger motor.
2. Description of the Background Art
A variety of continuously variable transmissions are known. Some hydraulic continuously variable transmissions, including a hydraulic pump and a hydraulic motor in combination, have been known and put to practical use.
One example of a commercial hydraulic continuously variable transmission was sold by Honda during the year 2001, and marketed under the trademark “Hondamatic” on the 2001 Honda Fourtrax Foreman Rubicon™ TRX500 ATV.
Another example of a known a hydraulic continuously variable transmission is disclosed in Japanese Patent Laid-open No. Hei 6-42446 and Japanese Patent No. 2920772 proposed by the present applicant. The hydraulic continuously variable transmission disclosed in these patent documents includes a swash plate plunger pump, a swash plate plunger motor, and a closed hydraulic circuit for connecting a discharge port and a suction port of the swash plate plunger pump, respectively, to a suction port and a discharge port, respectively, of the swash plate plunger motor.
In the transmission disclosed in Japanese Patent Laid-open No. Hei 6-42446 and Japanese Patent No. 2920772, a pump swash plate member is driven by an engine, a pump cylinder and a motor cylinder are connected by an output shaft, a motor swash plate is restricted in rotation, and the motor swash plate angle can be variably adjusted.
Further in the hydraulic continuously variable transmission disclosed in Japanese Patent Laid-open No. Hei 6-42446 and Japanese Patent No. 2920772, the pump cylinder and the motor cylinder are connected to each other in a back-to-back relationship, and pump and motor distribution valves (distributor valves) are disposed at the connected portion, constituting a closed hydraulic circuit.
The pump and motor distribution valves are structured so that hydraulic fluid, coming from a pump plunger which is reciprocated in a pump cylinder according to rotation of the pump swash plate, is supplied into a motor cylinder chamber to push a motor plunger, and the motor plunger is moved in the axial direction in sliding contact with the motor swash plate, thereby rotating a motor cylinder.
Meanwhile, in the hydraulic continuously variable transmission including the axial plunger pump as mentioned above, when the hydraulic fluid is compressed by being pushed by the plunger slidingly moved in the plunger hole in the compressing direction, there is the problem that if air has been mixed into the hydraulic fluid, the compression efficiency of the pump is lowered because air is highly compressible. Hitherto, particularly, since a pump discharge opening for discharging the hydraulic fluid from the plunger hole when the plunger is moved in the compressing direction is smaller in diameter than the plunger hole, there has been the problem that air would remain at a stepped portion between an end portion of the plunger and the pump discharge opening, resulting in a lowering in compression efficiency.
Although the known devices have some utility for their intended purposes, there is still a need to provide an improved hydraulic continuously variable transmission. More particularly, there is a need for an improved hydraulic continuously variable transmission having a structure in which hydraulic fluid discharged from an axial plunger pump is supplied to an axial plunger motor through a closed hydraulic circuit to drive the axial plunger motor to rotate.